I can see there are many ways to override built-in behaviors. But what happens if a package adds an override that I don't want? I'm thinking specifically of SublimeJedi, which steals ctrl+button1, which has the very important built-in behavior of adding a selection. Obviously, I can edit the package's Default.sublime-mousemap and remove the offending override, but then I have to do this every time the package updates. Seems like I need to write my own Default.sublime-mousemap in Packages/User to override the override. What should it say? I can't find any documentation for mousemap. And ST3 lets me view the default keymap, but not the default mousemap.

(It occurs to me that this particular case might be a bug in SublimeJedi. The mousemap I'm referring to reads

Yes you can likely override the file (as it's supported in ST3), but creating a "Default (<OS>).sublime-mousemap" is probably a better solution. You can see the default values in the Default folder. To view the defaults, without having to manually extract, take a look at PackageResourceViewer.